Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the free energy of austenitic grain boundaries for steels whose carbon content is equal to 0.2%, for chrome-manganese steels with various concentrations of molybdenum and phosphorus, and for chrome-manganese-molybdenum steel. On the basis of regression analysis, we develop an interpolational model which enables one to estimate (both qualitatively and quantitatively) not only the effects of each element on the free energy of grain boundaries but also changes in these effects caused by the presence of other elements with different surface activities. It is shown that changes in the influence of alloying elements on the boundary energy of multicomponent systems can be explained by the interaction of different elements. Philadelphia, USA. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 3, pp. 55–62, May–June, 1996.     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the free energy of boundaries of austenitic grains and analyze the effect of alloying elements (Mo, Ni, V, W, Nb, Ce, Cr, Ti, Al, Si, B, and Cu) on the boundary energy of low-carbon, chromemanganese, and chrome-manganese-molybdenum steels. By using regression analysis, we develop interpolational models for the qualitative and quantitative evaluation of the effect of each element on the free energy of grain boundaries. Philadelphia, USA. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 2, pp. 24–34, March–April, 1996.     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the conditional energy σ, of austenitic grain boundaries and analyze changes in boundary energy at elevated temperatures for low-carbon, chrome-manganese and chrome-manganese-molybdcnum steels. It is established that σ, varies within the range 1.0–1.3 J/m² and that, in almost all cases of alloying, the effect caused by a decrease in the surface energy of iron is so strong that even in the presence of chorophobic elements σ, decreases at elevated temperatures. For multicomponent steels, we analyze variations of the temperature coefficients of boundary energy. Philadelphia, United States of America. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 63–68, July–August, 1996     

Layering transitions at grain boundaries

We review various simplified models that have been advanced to describe layering (complexion) transitions at grain boundaries in multicomponent solids. In particular, we first outline lattice-gas, off-lattice atomistic and thermodynamic models that have been employed to investigate phase-like behavior at segregated grain boundaries. The results of these investigations are summarized in the form of complexion diagrams in different thermodynamic planes, and we highlight important features of these diagrams, such as complexion transition lines and critical points. Finally, we describe current issues and provide a future outlook.     

Solid-state wetting transitions at grain boundaries

A previously developed model of the dependence of grain boundary (GB) segregation on GB character has been exercised to investigate solid-state wetting transitions at GBs, and their anisotropy. In the case of binary systems displaying a solid-state miscibility gap, it is shown that the wetting transition temperature for precipitates at a GB is anisotropic, and is inversely related to GB energy. The model also allows calculation of prewetting transitions and associated excursions in adsorption off phase coexistence. These transitions are first order below a prewetting critical point (T_PWC), and higher order at temperatures above T_PWC. Investigation of the prewetting behavior provides the means for construction of the two-dimensional phase diagram of a GB.     

Diffusion mechanisms at metallic grain boundaries

Summary As the sizes of electronic devices continue to shrink, understanding key atomic phenomena vital to macroscopic processes become increasingly important. Mass transport along grain boundaries (GBs) is such a key process. We have studied diffusion mechanisms at metallic GBs of Ag and Al with the embedded-atom method, with molecular statics (MS) and molecular dynamics (MD), as well as with massively parallel computers at Oak Ridge National Laboratory and Sandia National Laboratory. Formation and migration energies of interstitials and vacancies at the optimal symmetric tilt GBs of different angles in Ag were first obtained using MS at 0 K. Extensive MD simulations were then carried out for selected Ag GBs using massively parallel computers at different finite temperatures up to the melting point. In this way, we were able to determine the dominant diffusion mechanism within different temperature regimes by comparison of the activation energies from MS results for the identified diffusion processes with the MD simulation results. For the first time, the results of this study on Ag GBs had provided realistic explanation and simulation-based evidence for the discrepancy between the activation energies from the recent low-temperature experiments by Ma and Balluffi and those at high temperatures reported in the literature. Preliminary MD results on Al GB diffusion are in excellent agreement with experiment and on-going work on Al and Al-Cu systems aimed to further understand electromigration phenomena will be briefly discussed.     

Effect of boron and sulphur on the electronic structure of grain boundaries in Ni

The first-principles discrete variational method is employed to study the effect of boron and sulphur on the electronic structure of the Ni grain boundary (GB). The calculated results show that boron does not strongly influence (only slightly decreases) the bonding between the atoms of the metal. In addition, B forms the strong bonding state with its neighbouring metal atoms. Our study also indicates that S strongly decreases the bonding between the atoms of the metal, and that the bonding tendency between S and the atoms of the metal across the GB plane is very weak. The calculations of environment-sensitive embedding energies show B has the strong site-competition ability and can successfully drive out S from the GB region. We conclude that the influence of impurities segregating on the GBs is closely associated with their effects on: (i) the decrease of the bonding between the atoms of the metal due to the presence of impurities; (ii) the bonding between the impurity atom and the atoms of the metal; and (iii) the site-competition ability of impurity atoms.     

Insights on slip transmission at grain boundaries from atomistic simulations

To fully understand the plastic deformation of metallic polycrystalline materials, the physical mechanisms by which a dislocation interacts with a grain boundary must be identified. Recent atomistic simulations have focused on the discrete atomic scale motions that lead to either dislocation obstruction, dislocation absorption into the grain boundary with subsequent emission at a different site along the grain boundary, or direct dislocation transmission through the grain boundary into the opposing lattice. These atomistic simulations, coupled with foundational experiments performed to study dislocation pile-ups and slip transfer through a grain boundary, have facilitated the development and refinement of a set of criteria for predicting if dislocation transmission will occur and which slip systems will be activated in the adjacent grain by the stress concentration resulting from the dislocation pile-up. This article provides a concise review of both experimental and atomistic simulation efforts focused on the details of slip transmission at grain boundaries in metallic materials and provides a discussion of outstanding challenges for atomistic simulations to advance this field.     

Local migration of grain boundaries in polycrystalline materials under plastic deformation conditions

We propose a theoretical model describing the local migration of grain boundaries (GBs) near triple junctions according to the new mechanism stimulated by the GB slip. Within the framework of this model, a driving force for the local migration is due to the interaction between sliding and structural GB dislocations responsible for the GB slip and misorientation, respectively.     

Grain boundary migration during abnormal grain growth in nanocrystalline Ni

The transformation mechanisms of abnormal grain growth in nanocrystalline Ni were studied extensively by transmission electron microscopy (TEM). A combination of in situ TEM annealing and ex situ annealing followed by TEM characterization was used. It was observed that grain boundary migration is both spatially and temporally non-uniform; migration occurs in a series of discrete steps, which are followed by periods of stagnation.